Sure, as long as you make a few changes; neuter the bayonet lug and permanently attach a brake over the threads.
For example I recently bought a complete pre-ban upper to use on a post-ban lower. I replaced the front sight base with a Bushmaster 3-rail base and permanently attached a CavComp over the threads. Voila! A legal post-ban.

Insanity in individuals is rare, but in groups, parties, and nations it is the rule. -- Nietzsche